Golf balls are required to meet aerodynamic symmetry as prescribed in Professional Golfers' Association Rule, for example, Japan Professional Golfers' Association Rule, Appendix III, Ball (C). It is required that when hit under given conditions, a ball give essentially no difference in trajectory and distance irrespective of different hitting positions.
Currently commercially available golf balls are considered to meet the required aerodynamic symmetry as long as the prescription is concerned, but tend to give a slight difference in trajectory and distance depending on a particular hitting position. If balls are aerodynamically asymmetric, then such asymmetry, though quite slight, would cause inconsistent shots especially for skilled players and professional golfers.
One of the causes for aerodynamically asymmetric golf balls is the presence of a seam line. Since golf balls are most often manufactured by compression molding in mated mold halves each having a semispherical molding surface, a seam line is formed at the mating line between the mold halves as a great circle with which none of the dimples intersect. Therefore, seam lines are inevitably present on golf balls for the manufacture reason.
The ball hitting test prescribed in Japan, UK or US Professional Golfers' Association Rule, Appendix III, Ball (C) is now described in detail. Referring to FIGS. 8 and 9, a golf ball a is shown as having a center d and a seam line b. Two hitting tests are prescribed, that is, a hitting test of FIG. 8 called pole hit where the golf ball a is hit at f so as to produce a back spin about a diametrical line e connecting three points, two diametrically opposite points c and c on the seam line b and the center d, and a hitting test of FIG. 9 called seam hit where the golf ball a is hit at i so as to produce a back spin about a diametrical line h extending perpendicular to a circular plane g having a circumference coincident with the seam line b and passing through the center d. The aerodynamic symmetry of the ball is evaluated in terms of differences in carry distance, peak angle (the angle of a straight line connecting the maximum point the ball reaches and the ground location where the ball is hit with respect to the horizontal line), and flight time between the two hitting tests. It is known for golf balls having a seam line that in these hitting tests, the balls given a pole hit assume a trajectory having a larger peak angle than in the case of a seam hit, resulting in a difference in carry distance and flight time.
It was proposed to improve the aerodynamic symmetry of a ball by providing on the ball surface a plurality of great circles which do not intersect the dimples as the seam line does not. One example is an icosahedral distribution which is achieved by equally dividing the ball surface into 20 triangles of a regular 20-sided (icosahedral) body and distributing dimples in each of the triangles. There were proposed several similar golf balls having a high degree of geometrical uniformity. Room for improvement is left in such geometrically uniform golf balls.